1. Field of the Invention
The present invention generally relates to an image forming apparatus of a type utilizing a roll of donor sheet and a receptor sheet in the form of a cut sheet and, more particularly, to a sheet transport mechanism used in the image forming apparatus of the type referred to above.
2. Description of the Prior Art
An apparatus for and a method of duplicating an image with the use of a roll of donor sheet and a receptor sheet in the form of a cut sheet are well known in the art. The roll of donor sheet is sensitive to rays of light and also to pressure and comprises a rolled form of a length of sheet having microcapsules coated on one surface thereof, each of said microcapsules encapsulating a mixture of photo-curable or photo-setting material with an image forming agent, whereas the receptor sheet is of a type having at least one surface coated with chromogenic developing material. The image duplicating method using the roll of donor sheet and the receptor sheet is performed in the following manner.
When a web of donor sheet is exposed to imagewise rays of light carrying an image of an original to be duplicated, a latent image is formed thereon which is a negative image corresponding in pattern to the image of the original. The term "negative image" used hereinabove and hereinafter is intended to means that some of the microcapsules situated outside the pattern of the imagewise rays of light projected onto the web of donor sheet are cured or hardened by exposure to light enough to be unable to rupture.
When this web of donor sheet having the latent image formed thereon is placed on the receptor sheet with the microcapsule coated surface thereof facing the surface of the receptor sheet which has been coated with the chromogenic developing material, and the both are subsequently pressed together by the application of a pressure, the remaining, uncured microcapsules on the web of donor sheet which represent the latent image are ruptured. Once the remaining, uncured microcapsules are ruptured, the image forming agents, which have been encapsulated in the ruptured microcapsules, flow over that surface of the receptor sheet and then react therewith to develop a colored positive image corresponding in pattern to the image of the original. An example of the combination donor sheet and receptor sheet are disclosed in, for example, U.S. Pat. No. 4,399,209 (corresponding to the Japanese Laid-open Patent Publication No. 58-88739, published in 1983).
Because the donor sheet is sensitive not only to external rays of light, but also to an external pressure, the donor sheet is housed in the form of a roll within a media cartridge for the purpose of protecting it from accidental application of light and pressure thereto. The cartridge accommodating the roll of donor sheet is generally so designed as to be selectively loaded into and removed out of the image forming apparatus.
As a matter of course, the roll of donor sheet within the cartridge has a leader sheet of sufficient rigidity secured to a radially outermost end of the rolled donor sheet and partially exposed to the outside from a slit defined in the cartridge. The leader sheet partially protruding outwards from the cartridge is used to secure the donor sheet therethrough to an automatic loading means, when the cartridge is placed inside the image forming apparatus, so that the web of donor sheet can be guided through a transport passage and past an image transfer station at which the image forming agents, which have been encapsulated in the ruptured microcapsules, can progressively flow over that surface of the receptor sheet to develop a colored positive image corresponding in pattern to the image of the original. An example of the cartridge containing the roll of donor sheet and the manner in which the web of donor sheet is guided are disclosed in, for example, U.S. Pat. No. 4,912,510, issued Mar. 27, 1990 (corresponding to the Japanese Laid-open Patent Publication No. 1-54445, published Mar. 1, 1989).
On the other hand, the receptor sheet is generally employed in the form of a cut sheet for reasons of convenience after the image duplication and/or some other reasons, and is usually supplied either manually or automatically from a paper supply unit accommodating a stack of receptor sheets. At the transfer station along the transport passage defined in the image forming apparatus, the web of donor sheet exposed to the imagewise rays of light during its movement in the transport passage is mated with the receptor sheet as the both are passed through a nipping area defined between a pair of pinch rolls so as to develop the colored positive image on the receptor sheet. Thereafter, the web of donor sheet which has been consumed is take up around a take-up spool disposed within the image forming apparatus whereas the receptor sheet having the colored positive image developed thereon is, after having been separated from the web of donor sheet by the action of a peeling shaft and a scraper both disposed downstream of the pinch rolls with respect to the direction of transport of the web of donor sheet, heat-treated in a bright finishing unit and is then ejected outwardly from a paper ejector unit in the image forming apparatus.
The prior art image forming apparatus of the type described above has a problem associated with the separation of the receptor sheet from the web of donor sheet which is effected after the image development. Specifically, the separation of the receptor sheet from the web of donor sheet at the peeling shaft downstream of the pinch roll pair cannot be accomplished satisfactorily and tends to be adversely affected by environmental conditions such as, for example, ambient humidity.
If the separation of the receptor sheet from the web of donor sheet does not take place satisfactorily, machine troubles would occur such as a paper jamming caused by the receptor sheet or an unwanted take-up of the receptor sheet together with the web of donor sheet.
Accordingly, the present invention has for its primary object to provide an improved image forming apparatus effective to accomplish a satisfactory separation of the receptor sheet from the web of donor sheet subsequent to the image development regardless of any environmental condition.
Another important object of the present invention is to provide a leader sheet coupled to the roll of donor sheet used in the image forming apparatus of the type referred to above.